Concrete for 20 × 18″ piers, 12″ deep
| 80 lb bags0.6 ft³ per bag | 65bags |
|---|---|
| 60 lb bags0.45 ft³ per bag | 87bags |
| 50 lb bags0.375 ft³ per bag | 104bags |
| Total volume1.44 yd³, includes 10% waste | 38.88ft³ |
| Per pierbefore waste | 1.77ft³ |
| Ready-mix | 1.44yd³ |
| Rebar (6 × #4 vertical)3″ cover top and bottom | 60linear ft |
| Forming tube18″ diameter | 20ft |
| Bearing areathe footprint carrying the load | 1.77ft² |
| Allowable loadat 1,500 psf presumptive soil — check yours | 2,651lb |
Notes
- At 1.44 yd³ a ready-mix delivery is usually cheaper than 65 bags once you count the hours of mixing. Most suppliers have a 1 yd³ minimum plus a short-load fee.
- A 18″ pier bears on 1.77 ft², which carries about 2,651 lb on 1,500 psf soil. That is the presumptive value the IRC allows when nobody has tested the soil; soft clay or fill can be a third of it, and the pier does not care how many bags went into it if the ground underneath moves.
- Depth is set by the frost line, not by the load. 12″ clears most of the southern and central US; the northern tier runs 48–60″. A pier poured above the frost line lifts every winter and takes the structure with it.
- Use a proper wet mix, not the dry-pour method that works for fence posts. This is a load-bearing element — dry-poured concrete cures unevenly and never reaches its rated strength.
- Rebar is 6 × #4 here, which is normal practice for deck and porch piers. Anything carrying a roof, a second storey or a beam splice wants a structural drawing rather than a rule of thumb.
The volume
A 18″ pier 12″ deep is 1.77 ft³ of concrete. Across 20 of them, with 10% added for the flare at the bottom of a drilled hole and the slight bulge of a forming tube, that is 38.88 ft³ — 65 bags of 80 lb mix, or 1.44 cubic yards.
Deck and porch piers use a normal wet mix, not the dry-pour method that works for fence posts. This is a load-bearing element, and dry-poured concrete never reaches its rated strength.
The number that actually decides the size
A 18″ pier bears on 1.77 ft² of soil. At the 1,500 psf the IRC lets you assume when nobody has tested the ground, that carries about 2,651 lb.
Bag counts are the easy half of this question. Soft clay or fill can be a third of that presumptive value, and the pier does not care how much concrete went into it if what is underneath moves.
Depth is set by frost, not by load
12″ clears the frost line across most of the southern and central US. The northern tier runs 48–60″. A pier poured above the frost line lifts every winter and takes the structure with it.
Steel
6 × #4 bars vertical, 60 linear ft in total, with 3″ of cover top and bottom. That is normal practice for deck and porch piers. Anything carrying a roof, a second storey or a beam splice needs a structural drawing rather than a rule of thumb.
Nearby sizes
Read more
- Concrete for a Sonotube: Volume, Bag Counts, and the Bell at the BottomThe volume of a cylinder is the easy part. What surprises people is how fast it grows with diameter, and what happens to a tube full of wet concrete that was not braced.
- Bags or a Truck — The Crossover Is About One Cubic YardThe arithmetic is not really about price. It is about how long a slab can sit half-poured before the joint between the halves becomes permanent.
- Rebar in a Slab — Spacing, Cover, and Why Position Matters MoreRebar laid on the ground and left there does nothing. It has to be held up into the concrete, and that is the step that gets skipped.
- Concrete Slab Cost by State — 400 ft², All 51The figures price a 4-inch slab on grade. A driveway wants six inches and rebar, and that is not a small adjustment.